Sure, we know it’s just a casting switch in real life, but what happens on 'Doctor Who' when its protagonist regenerates?

Whether its the end of a contract or a story arc, in the world of Doctor Who, all things are fleeting. Over the course of the show’s half-century run, the man who anchors the show, the time-hopping Time Lord also known as The Doctor, has found himself at the end of his life span on numerous occasions.

Whether he’s been mortally wounded or it was simply his time to go, at various times throughout the series, the Doctor has undergone a unique transformation that’s signaled the end of one era and the beginning of another. It’s regeneration, the legacy of the Time Lords and the means by which they extend their life. It’s also a fantastic narrative tool for a long-running series.

But, what happens when the Doctor regenerates? What effect does regeneration have on the soul of a being who finds himself housed in a completely different shell at the end of cataclysmic trauma? Can he possibly be the same person, or is he a new person, born as a result of the Doctor’s previous horrors?

The long and the short of it being, the Doctor can regenerate as many times as the show’s fans want him to, taking on more varied personas as he goes. As the Journal of Cell Biology wrote, “This imperfect process enables the series to continue with a new actor portraying the Doctor upon the departure of his predecessor.”

All the doctors, minus one

Sure, regeneration is one hell of a convenient way to replace the franchise’s star, but each new incarnation of the Doctor is marked with his own distinct personality and priorities.

Making the switch to a new person isn’t always a joyous occasion. Fans of the series have long discussed the tenth Doctor’s last words right before he disappeared in a flash of light.

It’s a troubling statement that casts some doubt as to how much of himself the Doctor takes with himself with each new version. Is the Doctor the same man when the lights go down?

What Effect Does Regeneration Have On the Doctor?

Before the tenth Doctor, no other incarnation had protested his regeneration. Fan theory states that the tenth Doctor’s unusual fear is a sign that he’s expecting his own final death, not another regeneration. That fear of the true unknown is what causes his ultimate reluctance. However, isn’t the tenth Doctor’s fear essentially well-founded? Once he regenerates into Matt Smith, isn’t Tennant essentially gone forever (well, at least until the 50th anniversary special)?

What it really boils down to is your concept of personal identity, or your notions of what makes a man himself. Traditionally, one’s personal identity is composed of three primary aspects, according to philosopher Michael Hand. Those bits are character, physicality, and memory. Unfortunately, the Doctor strikes out on two out of three of those categories.

As the Doctor regenerates from person to person, his character undergoes a transition of varying degree. Though sometimes more subtle than others, his personality is always distinct. In addition, the Doctor maintains no sense of consistent physicality since he’s a completely different physical person every time he regenerates. However, things get a little stickier when we get to his memory, as the Doctor does retain his memories of past events from body to body. Does memory alone a person make?

Let’s get to that first thing: the character. It’s telling that the Doctor only regenerates when he’s undergone severe trauma. According to Psychology Today, “unpleasant experiences produce permanent changes in the brain and corresponding shifts in intelligence, emotional reactivity, happiness, sociability, and other traits that used to be thought of as set for life.” In other words, human beings can undergo drastic personality changes as a result of trauma, so why shouldn’t the Doctor? It’s unpleasant, but it’s life.

A case could also be made for the physical change being not too big a deal. By the time the average human hits 30, a significant portion of their cells have been cast aside and replaced. In other words, on a cellular level, there’s very little in your body that’s the same as it was ten or twenty years ago. Couldn’t that same rule be easily applied to the Doctor so that technical physical continuity isn’t super important?

In the End, the Doctor is the Doctor

Theres a book out there called Doctor Who and Philosophy: Bigger on the Inside, in which several critics discuss the overarching importance of spatiotemporal continuity, the notion that you are always you in the same space at the same time, so you maintain your personal identity in spite of incremental change. Even though the Doctor can jump from time to time, he exists as the same entity in his own temporal space.

Even though the Doctor’s changes are radical, not incremental, they are still all part of the preservation of his being. When the glow recedes, the Doctor is still the same basic guy he was before he regenrated. He’s infallibly an enthusiastic learner, a towering intellect, a born leader, and a good man (well, good Time Lord).

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Though the process of regeneration is traumatic and transformative, it isn’t the sign of one alien’s death. It’s the sign of that alien’s rejuvenation, the continuation of his life. Regeneration is the Doctor’s response to change, the great inevitability. He may come out the other side a different man, but that’s nothing we haven’t all experienced in our own life.